Humanized immunodeficient mice which enables human cells or human tissues to be analyzed in vivo are regarded as laboratory animals that can be not only utilized as a research tool for drug discovery but expected to contribute to medicine as a useful tool that permits in vivo basic research such as analysis on the differentiation or functions of human cells by the transplantation of the human cells. Thus, an attempt has been made to develop more useful humanized immunodeficient mice over long years.
In 1962, nude mice deficient in thymus-derived T cells due to a lack of the thymus were found as immunodeficient mice. The mice had other immune cells, and normal human cells failed to be engrafted in the mice, though some human cancer cells were successfully engrafted. In 1980, Melvin Bosma et al. found immunodeficient SCID mice which were natural mutants of mice of the C.B-17 strain and exhibited severe combined immunodeficiency (SCID) mutation (see for example, non-patent document 1). The scid mutation takes an autosomal recessive mode of inheritance. The SCID mice achieved some positive results in such a way that the human fetal thymus could be transplanted under the renal capsule of the mice owing to the absence of T cells and B cells. However, the SCID mice did not improve the rate of human cell engraftment to an expected level.
In around 1980, Makino also found female individuals that manifested polyuria and strong positivity to urinary sugar among mice with cataract, and established mice designated as NOD (non-obese diabetes) mice because the symptoms thereof were similar to those of human type 1 diabetes mellitus (insulin-dependent diabetes mellitus) (see for example, non-patent document 2). NOD/scid mice which permitted engraftment of human cells at a rate higher than that of the SCID mice were developed by mating the NOD mice with the SCID mice. The NOD/scid mice exhibited decline in complement activity, macrophage functions, natural killer (NK) cell activity, etc. derived from the NOD strain, and did not reject transplanted human hematopoietic stem cells. However, problems were pointed out, such as engraftment efficiency that was not high, and the short lifespan of the mice.
Furthermore, the present inventors developed NOD/SCID/γcnull mice (also simply referred to as “NOG mice”) which lack both of functional T cell and B cell, exhibit decline in macrophage function, deletion of NK cell or NK activity, and decline in dendritic cell function, and have excellent engraftment of heterologous cell, by backcrossing C.B-17-scid mice with NOD mice and backcrossing the resulting mice with interleukin 2 receptor γ chain gene-knockout mice (see for example, patent document 1). The NOG mice reportedly have much higher engraftment of human cells or tissues than that of conventional mice, enable transplanted human stem cells to differentiate even into mature cells, and are useful in the generation of diverse humanized mouse models (see for example, non-patent document 3).
On the other hand, NK cells are regarded as one type of cytotoxic lymphocyte that works as a major factor for innate immunity. The NK cells can damage tumor cells or virally infected cells without antigen sensitization and have the property of expressing surface antigens including CD56. However, it has been reported that, in the NOG mice, efficiency of differentiation of transplanted cord blood-derived hematopoietic stem cells into human NK cells were low, and human NK cells were maintained only for a short period even when peripheral blood-derived human mature NK cells are transplanted.